1. Field of the Invention
The present invention relates to a flat light source device having a multi-colored luminescence which is suitable for a back illumination of a liquid crystal display (referred to as "LCD") device such as for use in a car speedometer.
2. Brief Description of the Prior Art
In recent years, a flat light emission device formed of a light-guiding plate combined with a fluorescent lamp having a cold cathode as a light source has frequently been employed as a light source device for illuminating an LCD display device. FIGS. 3A and 3B are views showing a schematic constitution of a conventional example of the flat light emitting devices mentioned above, wherein FIG. 3A is a plan view and FIG. 3B is a cross-sectional side view.
In FIGS. 3A and 3B, a numeric sign 11 stands for a light-guiding plate, a front surface of which is a planar-shaped light-emitting surface 12 and a rear surface of which is a tapered light-diffusing surface 13. Two cold cathode fluorescent lamps 15 and 16 are disposed adjacently to a side surface 14 of the light-guiding plate 11 wherein they are aligned in parallel to each other. Herein the fluorescent lamp 15 emits colored rays of light while another fluorescent lamp 16 emits white rays of light. The lights emitted from those fluorescent lamps 15 and 16 intrude either directly or being reflected from a reflecting plate 17 into an inside of the light-guiding plate 11 to be diffusibly reflected on a light-diffusing surface 13 toward various directions as can be seen from arrows shown in FIG. 3B. Those rays of light are then transmitted from the light-emitting surface 12 as the flat light emission being averaged in intensity.
The flat light-emitting source device constituted mentioned above can illuminate the LCD device for monochromatic (black and white) display use by a dichromatic illumination and, consequently, the two fluorescent lamps 15 and 16 having respectively different luminescent colors from each other are employed therein. Further, the various luminescent colors such as white, red, green, blue etc. are obtainable by blending fluorescent materials, which renders it capable to illuminate the LCD device in plurality of the colored rays.
However, to attain the multi-colored illumination in the conventional flat light source device mentioned above, it requires the constitution wherein a virtual plane defined by both central axes of the two fluorescent lamps orients perpendicular with respect to the light-emitting surface 12 of the light-guiding plate 11 so that dimensions of the device are enlarged in thickness. The dimensional enlargement of the light-guiding plate in thickness induces a problem that slenderization of the device turns incapable.